I have a Honda Odyssey EX-L 2005 with 143600 miles on the odometer. I purchased this car from a private party and he was the only owner of this car . This was registered in new jersey first for the first 7 years and then he moved to raleigh north carolina and got it registered . I got this car last year in nov 2022 . I got the engine mounts changed and the timing belt changed after driving for 1000 miles on my way to Myrtle beach i got the engine light on which after checking from the ODB-II scanning device got the error code as P0420 which i did a reset at that time without getting it fixed as i was on the highway and came back home after a month  driving it for a 1000 miles more i got the engine light back but this time it was a error code for the spark plug misfire and my mechanic got all the spark plugs and coil ignition changed and reset the error code after driving it for 1000 miles again the engine light came back with the P0304 error code and he says that it can be the EGR Valve which may be the issue as he already replaced the spark plugs and the ignition coil. Can you help me guide what could be the issue ? Let me know if you need any more details .